A copying machine or a printer of electrophotography system includes a toner sensor magnetically detecting the concentration or the amount of remaining toner in a development unit used for developing an electrostatic image formed in a photosensitive material. An example of such a toner sensor is disclosed in Japanese Patent Application Laid-Open No. 2001-165910. In the sensor disclosed in Japanese Patent Application Laid-Open No. 2001-165910, four coils are employed and then the toner concentration is detected by a differential transformer method.
Further, Japanese Patent Application Laid-Open No. 2009-31257 discloses a technique that a first oscillation circuit causing a phase shift in an oscillation wave in correspondence to an inductance change in a first sensing coil and a second oscillation circuit causing a phase shift in an oscillation wave in correspondence to an inductance change in a second sensing coil are employed and then the difference of the two phase shifts is obtained so that the state of a metal is detected.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. H11-142219, a toner sensor is known that detects the amount of remaining toner in a development unit by employing a piezoelectric element. In such a toner sensor, a piezoelectric element is caused to mechanically oscillate by application of an alternating voltage. Then, when a large amount of toner (powder) is in contact with the piezoelectric element, the oscillation is stopped. When the toner (the powder) decreases so that the contact area becomes small, the piezoelectric element mechanically oscillates. Then, an electromotive voltage caused by the oscillation of the piezoelectric element is used for detecting the amount of the toner (the powder).